1. Field of the Invention
The present invention addresses a problematic phenomenon in well fluids, such as drilling muds, for which the term "sag" has been coined. Sag occurs, for example, when circulation of the fluid is stopped for a period of time, e.g. when the drill string must be tripped from the well, and is caused by the resulting settling or stratification of the fluid whereby "heavy spots" develop. Sag can also involve movement or shifting of the fractions, particularly the "heavy spots," where components such as barite have become concentrated. Sag may not occur throughout an entire well, but nevertheless, its occurrence in even a small section of the well can cause the problems referred to below.
Such settling is not particularly problematic if the well is a vertical or near vertical. The magnitude of the problem is also relatively small if the well, or the section of the well in question, is nearly horizontal. However, if the well or a section thereof has a relatively high deviation angle (i.e. angle with respect to vertical), but falling well short of 90.degree., sag problems can become particularly severe. The advent and recent strides in extended reach drilling, which have resulted in relatively highly deviated wells, e.g. wells with deviation angles of 20.degree. or more, has brought sag problems currently into focus in the industry.
Among the problems caused by sag phenomena are sticking of drill pipe, difficulty in re-initiating and/or maintaining proper circulation of the fluid, possible loss of circulation and disproportionate removal from the well of lighter components of the fluid.
2. Description of the Prior Art
Prior efforts to control sag phenomena have included modification of muds or drilling fluids by altering parameters such as the yield point, which were believed to affect sag. However, the basis for such variations was mainly actual field experience which, because of the inability to know with certainty precisely what is occurring downhole, involved a certain amount of guess work.
Techniques have been developed for testing and/or analyzing other properties of well fluids, in a laboratory environment, but these were not intended to analyze sag, and none of them is completely satisfactory for that purpose.
More specifically, one of these techniques was used to test for a phenomenon known as "top oil separation" in invert emulsion drilling fluids. Top oil separation does not, to the inventors' knowledge, occur in actual downhole conditions, only in laboratories and the like, but is sometimes considered a factor which should be controlled out of an excess of caution. Top oil separation does not involve the true statification of the drilling fluid which occurs in connection with sag, but merely refers to the sweating or bleeding of a relatively thin layer of pure oil to the upper surface of a volume of mud, with the remainder of mud remaining more or less homogenous. In accord with this technique, samples of the mud were placed in test tubes which were heated in an oven, whereafter the amount of oil which had separated to the top was measured. Coincidently, in at least some such procedures, the tubes were placed in the oven at an angle, simply because the length of the tubes and the size of the oven did not permit upright orientation. The result was not necessarily a measure of sag, or true stratification, which may or may not have occurred in the test. Furthermore, the numerical result was merely a quantity or percentage of oil and was not related to time.
Other systems, known as "flow loops" have been devised for studying the circulation of cuttings. While it is conceivable that such systems might be utilized to test or predict sag, because they are really intended to analyze cutting circulation, and then due to the size of typical well cuttings, it has been felt, perhaps correctly, that the test systems should be approximately "full scale." Thus, they are large, expensive, and may require a relatively high degree of skill and/or training to operate. Furthermore, the flow loops are, by nature, intended to simulate and analyze a dynamic condition, i.e. circulation, and not the static condition, when circulation is lost or stopped, which results in sag.
Other methods have been used to test the shear strength of drilling fluids using conventional rheometers. However, this neither simulates nor accurately predicts sag.